1. Field of the Invention
This invention relates to rotational speed control systems for rotating members, and in particular, rotational speed control systems for rotating disk memory systems.
2. Background of the Invention
In various applications involving rotating members, and in particular rotating disk memory systems, it is important to maintain the rotational speed of the rotating disk at a specified value.
In many prior art rotating disk memory systems, and in particular in Winchester disk drive systems, the rotational speed of the rotating disk is controlled by a microprocessor which also performs numerous other control functions in the disk drive system. In such systems the microprocessor monitors index pulses derived from rotation of the spindle which supports the magnetic disks and compares the time between successive index pulses to a predetermined time period corresponding to desired rotational speed. The microprocessor thus determines whether the spindle is rotating too fast or too slow and provides appropriate command signals to the spindle drive motor to compensate for the speed error.
Although the monitoring of spindle rotational speed by the control microprocessor is effective, it ties up the microprocessor for a considerable percentage of the available time (commonly as much as one half). Therefore, a larger and more sophisticated and hence correspondingly more expensive microprocessor is required to handle the numerous other control functions along with the spindle speed control monitoring. Alternatively, two microprocessor chips may be required to provide adequate system control as opposed to a single one if the spindle speed monitoring function could be performed externally to the microprocessor.
Prior art approaches to controlling the rotational speed of a rotating member without the use of a microprocessor generally do not provide the high accuracy level of rotational speed control provided by microprocessor systems. A microprocessor control system has the capability of providing a speed up or slow down command to the drive motor after the occurrence of each index pulse and relating the magnitude of the command to the size of the speed error. Accurate speed correction can thus be accomplished for each revolution of the disk (assuming one index pulse per revolution). This type of rotational speed control may be referred to as frequency error detection.
Prior art non-microprocessor controlled speed regulating circuitry, however, does not provide this high accuracy level of error detection. One type of prior art rotational speed control circuit compares externally provided reference pulses with the index pulses and determines whether the index pulses occur before or after the reference pulses based purely upon a phase comparison at each index without resetting the reference counter after each comparison. This method, as frequently practiced in feedback control systems, is well known and generally referred to as a phase locked scheme. Such a scheme, although ultimately accurate, is generally more complex than the frequency dectector based concept of the speed control scheme of the present invention. Factors that render the phase detector scheme more complex include the difficulty of obtaining speed synchronism over a wide range of initial frequency error. While it is further recognized that frequency control schemes are generally well known in the art of control systems, it is the unique useage of the two-counter error detector which constitutes an important aspect of this invention. An example of a phase locked speed control scheme may be seen in U.S. Pat. No. 3,842,326 to Stocker for Velocity Control System for Reel-to-Reel Web Drive.
Some prior art rotational speed error detection circuits which reset the reference pulse for each index pulse lack the capability of giving a magnitude proportional to the speed error, and can only sense the direction of the speed error. Such a rotational speed control system is shown in U.S. Pat. No. 4,243,921 to Tamura et al.
Accordingly, it is an object of the present invention to provide a system for controlling the rotational speed of a rotating member, without utilizing a microprocessor other than for providing a readily available crystal controlled reference clock, which provides rotational speed control based on frequency error detection.
It is a further object of the present invention to provide a rotational speed control system for use with a rotating disk memory system which employs a relatively simple circuit configuration with corresponding low cost.